1. Field of the Invention
This invention relates to a high strength steel suitable for application as high strength bolts for motor vehicles or as hexagon socket head cap screw on various industrial machines, and more particularly to a high strength bolt steel which is improved in delayed fracture strength and cold forgeability.
2. Description of the Prior Art
Low-alloy steel for machine structural use, especially, AISI 4135 and 4140 are generally used for high strength bolts. These steels have the tensile strength of 120-130 kgf/mm.sup.2, and are endurable up to a considerably high stress. In a particular field of application where a higher stress in required, attempts have been made to achieve a required strength by modification of alloy elements.
However, such modified steels used for high strength bolts have the problem of the so-called delayed fracture, i.e., a sudden fracturing occures during use over a long period of time in fastenned state. In this regard, a number of laid-open patent applications disclose the results of researches and developments which have been conducted with a view to solving the just-mentioned problem. For example, Japanese Laid-Open Patent Application 60-114551 discloses steels achieving a high strength of the order of 140-160 kgf/mm.sup.2. This steel, however, is low hardenability due to suppression of Mn content to less than 0.40%, leaving problems regarding stability of the high strength and occurrence of increased surface defects attributable to insufficient deoxidation, accompanied by insufficient deformability in cold forging. The steel of the above-mentioned patent application has a Ti content greater than 0.05% for the purpose of improving the ductility through making austenitic crystal grains finer. The increased Ti content however is reflected by increasing of the precipitation of Ti oxides and nitrides which bring the improvement in the delayed fracture resistance. Japanese Laid-Open Patent Application 58-117858 discloses a steel which attains a high strength of the order of 130 kgf/mm.sup.2 through restriction of P and S contents, while attempting to improve the deoxidization by alloying Si of 0.1-0.8 %. This, however, impairs the cold forgeability, and induces the tendency toward production of intergranular oxides in the spheroidizing annealing to impede an improvement in the delayed fracture resistance.